Why are some corals flourishing in a time of global warming?

A new study investigates why gorgonian corals, which can form a ‘canopy’ over reefs, appear to be proliferating in certain places

Gorgonian coral-dominated coral reef communities at St. John, U.S. Virgin Islands. Stony corals are present but much of the three-dimensional structure of the reef community is created by the gorgonian corals. Credit: Howard Lasker

“Does this mean that as stony corals continue to decline, we’re going to see reefs transforming into these gorgonian coral-dominated communities? That’s what we’re trying to find out.”

Howard Lasker, professor of geology

University at Buffalo

Download High-Res Images

Pictured below are gorgonian coral-dominated coral reef
communities at St. John, U.S. Virgin Islands. Stony corals are
present but much of the three-dimensional structure of the reef
community is created by the gorgonian corals. Credit: Howard
Lasker

BUFFALO, N.Y. — As Earth’s temperature climbs, the
stony corals that form the backbone of ocean reefs are in
decline.

It’s a well-documented story: Violent storms and coral
bleaching have all contributed to dwindling populations, and
increasing acidity of seawater threatens to take an additional
toll.

Less discussed, however, is the plight of gorgonian corals
— softer, flexible, tree-like species that can rise up like
an underwater forest, providing a canopy beneath which small fish
and aquatic life of all kinds can thrive.

Divers have noted in recent years that gorgonian corals seem to
be proliferating in certain areas of the Caribbean, even as their
stony counterparts struggle.

Now, a new study will look to quantify this phenomenon.

Scientists from the California State University, Northridge and
University at Buffalo will examine 27 years of photographs from
reefs off the Caribbean island of St. John to determine how
gorgonian numbers have changed, and run field experiments to see
how competition with stony corals — or a lack of it —
influences gorgonian growth.

The study will also document what gorgonian coral populations
look like now at St. John, which is part of the U.S. Virgin
Islands, and track future development there.

Understanding coral reefs is important as they are one of the
planet’s most biologically diverse ecosystems.

“With climate change and ocean acidification, there
certainly is a realistic possibility that coral reefs as we know
them could pretty much disappear,” said Cal State Northridge
biology professor Peter Edmunds, one of two investigators heading
the project. “The question is, what will coral reefs look
like in the future?”

“When you look at these gorgonian corals, it seems that
they’re increasing in abundance, and that’s an
anecdotal observation that many people have made,” said UB
geology professor Howard Lasker, the other principal investigator.
“Does this mean that as stony corals continue to decline,
we’re going to see reefs transforming into these gorgonian
coral-dominated communities? That’s what we’re trying
to find out.”

The nearly $1 million project, funded by the National Science
Foundation (NSF), started officially on Sept. 1.

It brings together a powerful team. There’s Lasker, a
leading authority on the biology of Caribbean gorgonian corals, and
Edmunds, who studies the region’s stony corals and has
amassed an archive of photographs dating back to 1987 that document
changes on the reefs at St. John. A third partner is postdoctoral
researcher Lorenzo Bramanti, currently working in France at the
Université Pierre et Marie Curie (UPMC), who is an expert on
underwater communities in the Mediterranean where gorgonian corals
are dominant.

Corals, both gorgonians and stony corals, are aquatic animals
that assemble themselves into colonies formed from tiny individuals
called polyps.

Stony corals play an important role in reefs: When stony coral
polyps die, they leave behind a rock-like skeleton, which forms a
platform on which other life forms can grow, including new
generations of corals.

Gorgonian corals serve a different purpose. They form tree-like
colonies that give reefs 3-dimensional complexity, providing a
habitat for fishes and invertebrates, Lasker said. For this reason,
Bramanti and other Mediterranean researchers use a poetic
descriptor to refer to gorgonian-dominated communities:
“animal forests.”

Though gorgonians often grow on the remains of stony corals,
they can also grow on any solid surface. What the Buffalo and
California team is trying to figure out is how and why the balance
between gorgonian and stony corals is changing as the Earth
warms.

Preliminary data from Edmunds’ 27-year photo set suggests
that gorgonian coral density has indeed been increasing on the
shallow reefs surrounding St. John. The researchers need to take a
closer look at the archive, but pictures from five-year intervals
show a notable rise in gorgonians starting around 1997, even as
stony corals have declined.

The team’s field studies will try to explain why
gorgonians may flourish when stony corals are in decline. The
experiments, taking place in St. John’s waters, will look at
how two or three species of gorgonians fare when growing alongside
different combinations and densities of stony corals and algae.

Using the historical data and observations from the field, the
scientists hope to model how the reef — and others like it
— may look decades from now as the Earth’s climate
continues to shift.

The study will fill in important gaps in knowledge, said Lasker,
pointing out that gorgonian corals have historically drawn less
attention than their stony counterparts, in part because individual
species are extremely difficult to identify.

As their NSF project abstract states, “Reefs are more than
the (stony) corals and fishes for which they are known best, and
their biodiversity is affected strongly by other groups of
organisms.”

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